1. Field of the Invention
The present invention generally relates to a liquid crystal display device and its fabricating method, and more particularly, to a liquid crystal display device using ferroelectric liquid crystals.
Liquid crystal display devices are generally used as small-size and low-power consumption displays in small-size and low-cost portable information processing devices such as laptop types of personal computers and word processors. Because the liquid crystal display devices occupy little space, they are recently also being used in desktop types of the personal computers and the word processors.
2. Description of the Prior Art
A liquid crystal display device in conventional laptop types of personal computers and word processors mainly uses super twisted nematic (STN) liquid crystals. The STN liquid crystals can reversibly change their optical conditions according to an applied electric field, so that these crystals enable a gray-scale display with a halftone. The liquid crystal display device using the STN liquid crystals is generally constructed with a simple matrix driving method, which can display about 640.times.400 picture elements in a contrast ratio of about 10 to 1 at a relatively low voltage of about 20 V. However, there are problems with the simple matrix driving method when a display of a high information content, reaching to 1240.times.1028 thereof, which is required for a work station, is tried in the conventional liquid crystal display device. Since a driving voltage is directly supplied in sequence to each liquid crystal cell corresponding to a respective picture element through an X-electrode and a Y-electrode constructing a simple matrix in a time division method, a frequency of a driving pulse directly supplied to the liquid crystal cell becomes extremely high. There is thus a problem that a contrast between two adjoining picture elements is extremely degraded. And with this problem, a response speed and a visual field angle are also degraded such that it is difficult to satisfy a required display quality.
To overcome these problems, in a color liquid crystal display device having a higher information content, thin-film transistors (TFTs) are formed at cross points of the X-electrode and the Y-electrode constructing the simple matrix, and using the TFTs, the driving voltage is supplied to the liquid crystal cell corresponding to the respective picture element. This method is called an active matrix driving method. In the liquid crystal display device using the active matrix driving method, the TFTs in one X-line are selectively activated by an X-line selection pulse supplied to the X-electrode, and 1-bit image data supplied to the Y-electrode is written into each associated liquid crystal cell through the TFTs. In this way, in the active matrix driving method, the driving pulse supplied to the X-electrode and the Y-electrode does not directly drive the liquid crystal cell, but drives the liquid crystals individually through the TFTs. Therefore, this method is used for the color liquid crystal display devices having the high information content.
However, when constructing a large-size display device whose diagonal length is larger than, for example, 15 inches, which is to be used in the work station, an extremely large amount of TFTs need to be formed without a defect in the active matrix driving method. Therefore, there is a problem that a fabricating yield is degraded. And, in general, the TFTs, which are formed in a large area such as a face of the display device, are constructed with an amorphous silicon because of restrictions from semiconductor device fabrication technologies. In addition, since the amorphous silicon has a low electron mobility, there is also a problem that the response speed in the display device having the diagonal length more than 15 inches is degraded. In the present art, an application of the active matrix driving method is limited to a display device having a diagonal length less than 10 inches.
In this situation, a display device using ferroelectric liquid crystals is proposed for the liquid crystal display device having a large display area. The ferroelectric liquid crystals are materials having an optical bistability characterized in hysteresis. Therefore, by supplying a driving pulse similar to that used in the active matrix driving method to the liquid crystal cell through the X-electrode and the Y-electrode constructing the simple matrix, a desired optical condition may be maintained at a stable condition. Namely, by using the ferroelectric liquid crystals and a driving method similar to the conventional active matrix driving method, in a simple-configuration liquid crystal display device using the X-electrode and the Y-electrode constructing the simple matrix similar to that of the liquid crystal display device using the conventional STN liquid crystals, a high-resolution display of more than 1240.times.1028 picture elements in a wide area having the diagonal length larger than 15 inches may be realized with high quality and high stability. Further, the ferroelectric liquid crystals have advantages of a high response speed, a wide visual field angle, a high contrast, and a low cost.
As mentioned above, the liquid crystal display device using the ferroelectric liquid crystals is superior for the high-resolution display device of a high-performance information processing device. However, the ferroelectric liquid crystals are optical bistable materials, so that there is a problem that it is difficult to display the halftone.
To realize the gray scale display in methods except a halftone display in the ferroelectric liquid crystal display device, a variety of methods are proposed. For example, a single picture element is divided into a plurality of sub-picture elements, and by turning on/off these sub-picture elements, a gray-scale display may be realized. However, in this method, since the single picture element is divided into further elements, there is a problem that to realize the high-resolution display is difficult. On the other hand, even if the single picture element can be divided into the plurality of sub-picture elements, as a result of the division, a total number of the sub-picture elements becomes an extremely large number. Therefore, there is also a problem that it takes a long time to write image information on to the display. Further, for the extremely high information content, a large number of driving circuits need to be installed, so that there is a problem that it is difficult to physically connect the driving circuits with the X- or Y-electrode.
Also, other methods are proposed (for example, M. Kimura, et al., Proc. SID, vol.31/2, pp.139-143. 1990, and A. G. W. Verhulst, Proc. SID, vol.32/4, pp.379-386. 1991). In each of these methods, a domain, in which polarizations are reversed each other, is formed in the single picture element, and by controlling this domain, the gray-scale display in the ferroelectric liquid crystal display device may be realized without increasing a number of the picture elements. However, in these methods, it is difficult to always form the domains in a uniform density when there are more than one million picture elements, which is a number of picture elements necessary for the work-station display. And, a producing process of the domain is very sensitive to a temperature, there is thus a problem that a temperature control of the display is indispensable.
Further, yet another method is proposed as a modification of the method that the gray-scale display is realized by controlling the domain inside a ferroelectric liquid crystal layer. In this method, a molecular orientation film contacting with the liquid crystal layer is formed of a conductive material, and thus a reverse field generated with a reversing of the polarization in the domain disappears. Thus, the reversing of the polarization is hastened. However, when the molecular orientation film is formed of the conductive material, there is a substantial danger that the X-electrode and the Y-electrode are shorted to each other. And there are further problems that it is difficult to always produce the uniform domains, and the producing process of the domains is unstable to the temperature.
As mentioned above, when the gray-scale display is tried in the ferroelectric liquid crystal display device without using the halftone display, there are a variety of problems.